Rolling mill gauge control method and apparatus including x-ray correction

ABSTRACT

An automatic gauge control is disclosed to provide on line control of the delivery gauge or thickness from at least one roll stand of a rolling mill. The gauge error of the workpiece strip leaving that one roll stand is determined to include an X-ray gauge error portion and is corrected by predetermined adjustment of that one roll stand to provide a desired gauge correction in relation to that one roll stand.

United States Patent 1191 1 1111 3,802,235

Fox 1 Apr. 9, 1974 I5 ROLLING MILL GAUGE CONTROL 3,561.237 2/1971 Eggerset al. 72/7 M H AND APPARATUS INCLUDING 3,625,037 12/ 1971 Michel 72/21X X-RAY CORRECTION [75] Inventor: Richard Q. Fox, Pittsburgh, Pa.Primary Emmi"er Miltn Attorney, Agent, or Firm-R. G. Brodahl [73]Assrgnee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

22 Filed: Nov. 6, 1972 ABSTRACT [21] Appl. No.: 303,724 An automaticgauge control is disclosed to provide on line control of the deliverygauge or thickness from at least one roll stand of a rolling mill. Thegauge error (gl. of the workpiece Strip leaving that one to stand is E58] d 72/6 9 16 termined to include an X-ray gauge error portion and oearc I is corrected by predetermined adjustment of that one roll standto provide a desired gauge correction in re- [56] SSE T lation to thatone roll stand.

3,328,987 7/1967 Ferace 72/8 10 Claims, 6 Drawing Figures SCREWDOWNSCREWDOWN DETECTOR DETECTOR I I 33 a SCREWDOWN SCREWDOWN POSITION -CPOSITION 1-- "2/ F\ FMILL ENTRY SIDE 25 GSIDE I r" t Q TEMPERATUREGUARDS 21 UARDS l l o GAUGE 3 DETECTOR 37 I 1 47 g 5 LOAD 1 LOAD I CELLICELL 27 as I s21 43 5] SPEED A sea spasm -BTE-sIQN- 'r BITENSION- 4!CONTROL i CONTROL INFORMATION II wer J I DEV CES- CONTROL L' SYSTEM 5|iO ERATOR STATION l I DISPLAY \JNITSt 1 CONTROL PANEL YIPR|NTOUET%EVICES.

WENIEDAPR 91914 3,802,235

' SHEET 2 UP 4 g l I ASD E K-Q AF FIG.2 J 6 |oo\ I 102 (I I v I IUNLOADED ou. DELIIVERY ENTRY OPENING GAUGE GAUGE (SDREF) ("0) (HE) I lASD lO4y I :02 2 F,IG.3

3 II E 1 1 I00 Lgssaa SCREWDOWN a. k REFERENCUSDREH- PRESENT GAUGE(HX)PRESENT SCREWDOWN(SD)- DESIRED GAUGE (H FIG.4

LOCK-ON SCREWDOWMLOSD) PRESENT SCREWDOWN (SD) L PRESENT GAUGE( HX)LOCK-ON GAUGE( HD) EATENTED APR 9 I974 SHEET 3 BF 4 ROLLING MILL GAUGECONTROL METHOD AND APPARATUS INCLUDING X-RAY CORRECTION CROSS REFERENCETO RELATED APPLICATIONS Reference is made to the following previouslyfiled and s ated,ra smamisatmwhiq is.assi sitwg Reference is made to thefollowing concurrently filed, on Nov. 6, 1973, and related patentapplications which are assigned to the present assignee:

U.S. Pat. Ser. No. 303,723 entitled Rolling Mill Gauge Control Methodand Apparatus Including Temperature and Hardness Correction and filed byA. W. Smith.

US. Pat. Ser. No. 303,721 entitled Rolling Mill Gauge Control Method andApparatus Including Entry Gauge Correction and filed by A. W. Smith andR. Q. Fox.

U.S. Pat. Ser. No. 303,725 entitled Rolling Mill Gauge Control Methodand Apparatus Including Speed Correction and filed by R. Q. Fox.

U.S. Pat. Ser. No. 303,722 entitled Roling Mill Gauge Control Method andApparatus Including Feedback Correction and filed by R. 0. Fox and D. J.Emberg.

U.S. Pat. Ser. No. 303,726 entitled Rolling Mill Gauge Control Method'and Apparatus Including Plasticity Determination and filed by R. Q.Fox.

BACKGROUND OF THE INVENTION The present invention relates to workpiecestrip metal tandem rolling mills and more particularly to roll forcegauge control systems and methods used in operating such rolling mills.

In the operation of a metal or steel reversing or tandem rolling mill,the unloaded roll opening and the speed at each tandem mill stand foreach reversing mill pass are set up to produce successive workpiecestrip or plate reduction resulting in work product at the desired gauge.Generally, the loaded roll opening at a stand equals the stand deliverygauge or thickness on the basis of theusual assumption that there islittle or no elastic workpiece recovery.

Since the operator provided initial roll opening setup conditions, orthe initial roll opening settings provided by an associated digitalcomputer control system operative with model equation inforamtion tocalculate the setup screwdown schedules for the respective stands of therolling mill, can be in error and since in any event certain millparameters affect the sand loaded roll opening during rolling and aftersetup conditions have been established, a stand automatic gauge controlsystem is employed if it is necessary that the stand delivery gauge beclosely controlled. Thus, at the present state of the rolling mill art,and particularly the steel rolling mill art, a stand gauge controlsystem is normally used for a reversing mill stand and for predeterminedstands in tandem rolling mills.

The well known gaugemeter or roll force system has been widely used toproduce stand gauge control in metal rolling mills and particularly intandem hot steel strip rolling mills and reversing plate mills whereexperience has demonstrated that roll force control is particularlyeffective. Earlier publications and patents such as an article entitledInstallation and Operating Experience with Computer and Programmed MillControls by M. D. McMahon and M. A. Davis in the 1963 Iron and SteelEngineer Year Book at pages 726 to 733, an article entitled AutomaticGage Control For Modern Hot Strip Mills by J. W. Wallace in the December1967 Iron and Steel Engineer at pages to 86, U.S. Pat. No. 3,561,237issued to Eggers et a1. and U.S. Pat. No. 2,726,541 issued to R. B. Simsdescribe the theory upon which operation of the roll force and relatedgauge control systems are based. Attention is also called to U.S. Pat.Nos. 3,568,637, 3,574,279, 3,574,280 and 3,600,920 issued to'A. W.Smith, which relate to roll force automatic gauge control systems. Inreferencing prior art publications or patents as background herein, norepresentation is made that the cited subject matter is the bestteaching prior art.

Briefly, the roll force gauge control system uses Hookes law incontrolling the screwdown position at a rolling stand, i.e., the loadedroll opening underworkpiece rolling conditions equals the unloaded rollopening or screwdown position plus the mill stand spring stretch causedby the separating force applied to the rolls by the workpiece. To embodythis rolling principle in the roll force gauge control system, a loadcell or other force detector measures the roll separating force at eachcontrolled roll stand and the screwdown position is controlled tobalance roll force changes from a reference value and thereby hold theloaded roll opening at a substantially constant value. I-lot strip millautomatic gauge control (AGC) including evaluation of roll forcefeedback infonnation involves the combination of a number of processvariables, such as roll force, screw position, and mill spring which areall used to evaluate the gauge of the strip as it is worked in eachstand. In addition, an X-ray gauge is used on the strip as it passes outof .the last stand to evaluate the absolute strip gauge produced.

The two gauge error detection system that are commonly used are theX-ray and roll force. X-ray gauges can be placed between each stand, butthey are expensive, difficult to maintain, and can detect errors only asthe strip passes between stands. The roll force error detection systemis much less expensive, and can be more easily implemented in relationto the operation of all stands, to detect errors in gauge as the strippasses between the rolls of a particular roll stand, providing immediateevaluation of desired corrections to the roll openings. The roll forcesystem, however, provides only a relative evaluation of the gauge, sinceit measures the amount of gauge deviation from a reference gauge, suchas the gauge at the head end of the strip A practical combination of thetwo systems uses rollforce feedback to calculate fast corrections tofluctuations on gauge, and an X-ray to evaluate the absolute gauge ofthe strip coming out of the last stand. The fast corrections arecalculated from the roll force feedback, the stand screwdown position,and the modulus of elasticity of the rolling stand. The slower X-raygauge evaluation calculates simultaneous corrections to several stands,so that the absolute balue of the gauge may be brought to the desiredvalue.

The output of both of these systems is a change in the positionreferences supplied to the screwdowns of selected roll stands.

The following well known formula expresses the basic roll force gaugecontrol relationship:

where:

h loaded roll opening (workpiece delivery gauge G or thickness) SDunloaded roll opening (screwdown position) K stand mill spring constantF stand roll separating force. Typically, the roll force gauge controlsystem is an analog arrangement including analog comparison andamplification circuitry which responds to roll force and screwdownposition signals to control the screwdown position and hold thefollowing equality:

ASD=AF*K where:

A F,= measured change in roll force from an initial force A SDcontrolled change in screwdown position from an initial screwdownposition. After theunloaded roll opening setup and the stand speed setupare determined by the mill operator for a particular workpiece pass orseries of passes, the rolling operation is begun and the ,screwdowns arecontrolled to regulate the workpiece delivery gauge from the reversingmill stand or from each roll force controlled tandem mill stand. Bysatisfying Equation (2), and the assumptions implicit in Equation (1),the loaded roll opening H in Equation (1) is maintained constant ornearly constant.

As the head end of the workpiece strip enters each roll stand of themill, the lock-on screwdown position LOSD and the lock-on rollseparating force LOF are measured to establish what strip delivery gaugeG rolling operation proceeds, the roll stand separating force F and theroll stand screwdown position value SD are monitored periodically andany undesired change in roll separating force is detected andcompensated for by a corresponding correction change in screwdownposition. The lock-on gauge LOG is equal to the lockon'screwdown LOSDplus the lock-on force LOF multiplied by the mill stand spring modulusK. The workpiece strip delivery gauge G leaving the roll stand at anytime during the rolling operation is in accordance with above Equation(1) and is equal to the unloaded screwdown position SD plus the rollseparating force F multiplied by the mill spring modulus K. The rollforce determined gauge error GE in relation to a particular roll standis derived by subtracting the lock-on gauge LOG from the delivery gaugeG. The following Equations 3, 4 and 5 set forth these relationships.

LOG LOSD K*LOF Sleuths aint i ds tqfthat 911593. 9- s the n.

G SD K*F GE G LOG [SD LOSD] (FLOF)*K To provide steady state gauge errorcorrection, the

, well known X-ray monitor gauge control system is usually employed toproduce screwdown offset for the roll force control. in the monitorsystem, an X-ray or other radiation gauge sensing device is placed atone or more predetermined process points and usually at least at aprocess point following the delivery end after the last roll stand ofthe mill in order to sense actual delivery gauge after a workpiecetransport delay from the point in time at which the actual deliverygauge is produced at the preceding stand or stands. The monitor systemcompares the actual delivery gauge with the desired delivery gauge anddevelops an X-ray gauge error as an analog feedback control signal toadjust the operation of the reversing mill roll force gauge controlsystem or one or more predetermined tandem mill stand roll force gaugecontrol systems to supply desired steady state mill delivery gauge. inthis manner, the conventional monitor system provides for transportdelayed correction of steady state gauge errors which are caused orwhich are tending to be caused by a single mill variable or by acombination of mill variables.

it is known in the teachings of the prior art that the mass flow volumeof material passing through the multiple stands of a tandem rolling millremains substan tially constant, such that the following relationship issatisfied:

where:

G is the delivery gauge or thickness of the material leaving a givenroll stand. I S is the workpiece speed leaving that same roll stand. Wis the width of the workpiece leaving that same roll stand. Since theworkpiece width remains substantially constant during the passagethrough the rolling mill, this mass flow relationship becomes:

When the last stand delivery gauge XG(LS) is measured by an X-ray gauge,this enables a mass flow delivery gauge to be established for an earlierstand N, as follows:

where:

XG(N) is the calculated mass flow delivery X-ray gauge leaving stand N.XG(LS) is the X-ray measured delivery gauge leaving the last stand.

S(N) is the measured speed of the workpiece leaving stand N.

S(LS) is the measured speed of the workpiece leaving the last stand.

It is known in the teachings of the prior art to establish an offsetcorrection for the screwdown positioning mechanism of stand N inaccordance with the difference between the desired reference exit ordelivery gauge at the last stand and the X-ray exit or delivery gauge atthe last stand as follows:

SD Offset(N) (desired exit gauge at last stand minus X-ray gauge at laststand) S(LS )/S(N) In operator controlled mills, some steady state gaugecorrecting operations can eventually be taken off the monitor system byscrewdown recalibration, and the like. between similar workpiece passesif steady state gauge error tends to exist along the entire workpieceand persists from workpiece to workpiece. In this manner, some reductionis achieved in the length of off gauge workpiece material otherwiseassociated with monitor transport delay. Similarly, corrective monitorsystem operation caused by head end gauge errors can be reduced bychanges in the operator or associated computer control system providedsetup from workpiece to workpiece.

A background general teaching of stored program digital computer controlsystem operation is set forth in a book entitled Electronic DigitalSystem by R. K. Richards and published in 1966 by John Wiley and Sons.

An additional detailed description of computer programming techniques inrelation to the control of metal rolling mills can be found in anarticle in the Iron and Steel Engineer Yearbook for 1966 at pages 328through 344 entitled Computer Program Organization For an AutomaticallyControlled Rolling Mill by John S. Deliyannides and A. H. Green, and inanother article in the Westinghouse Engineer for January 1965 at pages13 through 19 and entitled Programming For Process Control by P. E.Lego.

A'programmed digital computer system can be employed to make the gaugeerror ccgregtiorrscrewdown SUMMARY OF THE INVENTION In accordance withthe broad principle of the present invention, a system and method forcontrolling workpiece delivery gauge in a metal rolling mill in relationto both the roll force determined exit gauge error in the workpiecedelivered from a given roll stand and the determined X-ray gauge errorin that workpiece leaving the same roll stand, and controlling thescrewdown position of that one roll stand of the mill for correcting thedelivery gauge in relation to that given roll stand.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows a schematic diagram of atandem hot steel strip rolling mill and an automatic gauge controlsystem arranged foroperation in accordance with the present invention;

FIG. 2 illustrates the typical mill spring curve and workpiece reductioncurve for a given rolling mill stand and the operation of that rollstand for reducing the gauge of a workpiece passed through the rollstand;

FIG. 3 illustrates, in relation to the mill spring curve and theworkpiece reduction curve, the effect of a correction made to thescrewdown position setting for changing the unloaded roll opening of aroll stand to provide a desired change in the workpiece gauge deliveredfrom that roll stand;

FIG. 4 shows an illustrative gauge error detection operation in relationto the initial lock on conditions at the head end of the workpiece;

FIG. 5 shows a schematic illustration of the gauge error correctionoperation in accordance with the present invention; and

FIG. 6 shows an illustrative logic flow chart of a suitable gauge errorcorrection control program operative in accordance with the presentinvention.

DESCRIPTION OF THE GAUGE CONTROL SYSTEM AND ITS OPERATION There is shownin FIG. 1 a tandem hot strip steel finishing mill 11 operated withimproved gauge control performance by a process control system 13 inaccordance with the principles of the invention. Generally, however, theinvention is applicable to various types of mills in which roll forcegauge control is employed.

The tandem mill 11 includes a series of reduction rolling stands withonly two of the stands S1 and S6 shown. A workpiece l5 enters the mill11 at the entry end in the form of a bar and it is elongated as it istransported through the successive stands to the delivery end of themill where it is coiled as a strip on a downcoiler 17. The entry barwould be of known steel grade class and it typically would have a knowninput gauge or thickness of about 1 inch and a width within some limitedrange such as 20 inches to inches. The delivered strip would usuallyhave approximately the same width and a thickness based upon theproduction order for which it is intended.

In the reduction rolling process, the successive stands operate atsuccessively higher speeds to maintain proper workpiece mass flow. Eachstand produces a predetermined reduction or draft such that the totalmill draft reduces the entry bar to strip with the desired gauge orthickness.

Each stand is conventionally provided with a pair of backup rolls l9 and21 and a pair of work rolls 23 and 25 between which the workpiece 15 ispassed. A large DC drive motor 27 is controllably energized at eachstand to drive the corresponding work rolls at a controlled speed.

As previously described, the sum of the unloaded work roll opening andthe mill stretch substantially defines the workpiece gauge deliveredfrom any particular stand in accordance with Hookes law. To vary theunload work roll opening at each stand, a pair of screwdown motors 29(only one shown at each stand) position respective screwdowns 31 (onlyone shown at each stand) which clamp against opposite ends of the backuprolls and thereby apply pressure to the work rolls. Normally, the twoscrewdowns 31 at a particular stand would be in identical positions, butthey can be located in different positions for strip guidance duringthreading, for flatness or other strip shape control purposes orpossibly for another purposes.

A conventional screwdown position detector or encoder 33 provides anelectrical signal representation of screwdown position at each stand. Toprovide an absolute correspondence between the screwdown position andthe unloaded roll opening between the associated work rolls, a screwdownposition detection system which includes the screwdown positiondetectors 33 can be provided and calibrated from time to time.

Roll force detection is provided at each of predetermined stands by aconventional load cell 35 which generates an electrical analog signal inaccordance with the stand roll force. At the very least, each roll forcecontrolled stand is provided with a load cell 35 and in many casesstands without roll force gauge control would also be equipped with loadcells. Th number of stands to which roll force gauge control is appliedis predetermined during the mill design in accordance withcost-performance standards, and increasingly there is a tendency toapply roll force gauge control to all of the stands in a tandem hotstrip steel mill. In the present case, a roll force gauge control systemis assumed to be employed at each of the stands.

Conventional motorized sideguards 37 are located at predetermined pointsalong the mill length. The sideguards are operated during mill setup onthe basis of the widths of the upcoming workpiece thereby defining thesides of the workpiece travel path for guidance purposes.

The process control system 13 provides automatic control for theoperation of the tandem mill 11 as well as desired control forassociated production processes (not indicated) such as the operation ofa roughing mill. The process control system 13 can include a programmedprocess control digital computer system which is interfaced with thevarious mill sensors and the various mill control devices to providecontrol over many of the various functions involved in operating thetandem mill 1]. According to user preference, the control system 13, canalso include conventional manual and/or automatic analog controls forselected process control functions.

On the basis of these considerations, automatic gauge control system 39can include a digital computer systern operative to provide thefinishing mill on-line roll force gauge control function, such as aProdac 2000 (P2000) sold by Westinghouse Electric Corporation. Adescriptive book entitled Prodac 2000 Computer Systems Reference Manualhas been published in 1970 by Westinghouse Electric Corporation and madeavailable for the purpose of describing in greater detail this computersystem and its operation.

There is disclosed in the above referenced previously filed US. Pat.application Ser. No. 215,743 the logic flow chart of an illustrativeautomatic gauge control suitable for operation with the X-ray correctionoperation of the present invention. It should be readily understood bypersons skilled in this art tha the present invention is also suitablefor operation with other well known automatic gauge control systems forcontrolling the delivery gauge of a workpiece strip passed through atleast one stand of a rolling mill.

The digital computer processor can be associated with well knownpredetermined input systems typically including a conventional contactclosure input system which scans contact or other signals representingthe status of various process conditions, a conventional analog inputsystem which scans and converts process analog signals, and operatorcontrolled and other information input devices and systems 31 such aspaper tape teletypewriter and dial input systems. it is noted that theinformation input devices 41 are generally indicated by a single blockin FIG. 1 although different input devices can and typically would beassociated with the control system. Various kinds of information areentered into control system through the input devices 41 including, forexample, desired strip delivery gauge and temperature, strip entry gaugeand width and temperature (by entry detectors if desired), grade ofsteel being rolled, plasticity tables, hardward oriented programs andcontrol programs for the programming system, and so forth. The principalcontrol action outputs from the automatic gauge control or AGC systeminclude screwdown positioning reference commands which are applied torespective screwdown positioning controls 55.for operating the screwdownmotors 29 for screw movement, and speed control signals which areapplied to the respective speed and tension control system 53 to cause achange indrive speed to compensate for a change in thickness being madeby a screwdown movement.

Display and printout devices 51 such as numeral display, tape punch, andteletypewriter systems can also be provided to keep the mill operatorgenerally informed about the mill operation and in order to signal theoperator regarding an event or alarm condition which may require someaction on his part. The printout devices are also used to log mill dataaccording to computer log program direction.

Generally, the AGC system uses Hookes law to determine the total amountof screwdown movement required at each roll force controlled stand atthe calculating point in time for roll force and gauge error correction,i.e., for loaded roll opening and stand delivery gauge correction to thedesired value. The calculation defines the total change in the unloadedroll opening required to offset the gauge error causing condition.

During rolling operation, the on line gauge control system operates thestands to produce strip product having desired gauge and propershape,.i.e., flat with slight crown. On linegauge control is produced bythe roll force gauge control loops at the stands and the previouslynoted X-ray monitor gauge control system.

In the monitor system, the X-ray gauge 47 produces the X-ray gauge erroror deviation signal which indicates the difference between actual stripdelivery thickness and desired or target strip delivery thickness. Inother cases, it may be desirable to employ an absolute thicknessmeasurement X ray gauge signal to form a basis for monitor controlactions or, more generally, for screwdown offset control actions.

To effect on line gauge control in the closed loops, the AGC systemoperates at predetermined time periods such as every 2/10 second withthe screwdown position detector and load cell provided signals from eachstand as well as the X-ray gauge error signal to determine therespective stand screwdown adjustment control actions required forproducing desired strip delivery gauge.

In FIG. 2, linear approximations of the roll stand characteristic curvesas shown to illustrate the application of Hookes law to a rolling millstand and to illustrate the basis upon which the on line gauge controlsystem provides improved gauge control, accuracy and stability and otheroperating benefits. A mill modulus characteristic or mill spring curve100 defines the separation between a pair of workpiece reducing millstand work rolls as a function of separating force and as a function ofscrewdown position. The slope of the mill spring curve 100 is the wellknown mill spring modulus or constant K which is subject to variation aswell known to persons skilled in the art. When a correct screwdowncalibration is known and the screwdowns are positioned such that theempty work rolls are just facing, the unloaded screwdown zero positionis defined. The workpiece deformation characteristic or reduction curve102 is shown. The entry gauge H of the workpiece passed through the rollstand is reduced to the indicated delivery gauge H,, as defined by theintersection of the mill spring curve 100 and the product reductioncurve 102 to establish the stand roll force required for the indicatedoperation. The unloaded roll opening, sometimes called the screwdownbecause of the screw and nut system used for adjusting the roll opening,is the gauge that would be delivered if-there were no roll separatingforce. As the force increases with a constant roll opening, the deliverygauge increases, since the mill deflects as shown by the mill springcurve 100. If no force was exerted on the product being rolled, thegauge would not be reduced and the delivery gauge would be equal to theentry gauge. When the roll force increases, the product is plasticallydeformed and the delivery gauge decreases. The slope of the mill springcharacteristic line is called the mill modulus (K) and the slope of theproduct reduction characteristic is called the product plasticity (P).The delivery gauge is determined by the equilibrium point at which theforce exerted by the mill is equal to the force required to deform theproduct. Changes in entry gauge and product hardness result in a changein roll force and delivery gauge. The automatic gauge control moves thescrewdown to correct for these gauge changes. The main advantage of theroll force gauge control system is its ability to detect changes ingauge the instant they take place, as the product is being rolled in thestand. A shift in delivery thickness can be caused by a change in entrythickness or a change in hardness (usually caused by a change intemperature). This change in delivery gauge is immediately detected bymonitoring the roll separating force of the roll stand.

When the screwdowns are opened (positive movement) the unloaded rollopening increases as reflected by a change to the right in the graphicallocation of the mill spring curve 100 such that the theoretical springcurve intersect equals the new unloaded roll opening. With screwdownclosing the mill spring curve is shifted to the left in a similarmanner.

At any particular screwdown position and with correct screwdowncalibration, the stand workpiece delivery gauge I-I equals the unloadedroll opening as defined by the screwdown position SDREF plus the millstretch (F*K) caused by the workpiece. If the screwdown calibration isincorrect, i.e., if the number assigned to the theoretical roll facingscrewdown position is something other than zero because of roll crownwear or other causes, the stand workpiece delivery gauge H then equalsthe unloaded roll opening plus the mill stretch, plus or minus thecalibration drift.

The amount of mill stretch depends on the product deformationcharacteristic or reduction curve 102 for the workpiece. As shown inFIG. 2, the reduction curve 102 for a strip of predetermined widthrepresents the amount of force F required to reduce the workpiece fromthe stand entry gauge (height) H The workpiece plasticity P is the slopeof the curve 10 2,and the curve 102 is shown as being linear although asmall amount of nonlinearity would normally exist.

Desired workpiece delivery gauge H D is produced since the amount offorce F required to reduce the workpiece from H to H is equal to theamount of roll separating force required to stretch the rolls to aloaded roll opening H i.e., the intersection of the mill spring curve100 at an initial screwdown opening SDREF indicated by mill spring curve100 and the workpiece reduction curve 102 lies at the desired gaugevalue H As shown in FIG. 3, if the actual stand present gauge Hx is notthe same'as the desired gauge H there is a gauge error GE to becorrected. This condition can be corrected by changing th providedscrewdown position reference SDREF to the stand, such that a new millspring curve 104 becomes operative to result in the desired gauge Hbeing delivered from the roll stand and the gauge error GE is nowremoved.

It is known in accordance with the teachings of above referenced US.Pat. No. 3,561,237 that the required corrective screwdown adjustment ASD to correct a stand delivery gauge error GE is equal to the product ofthat gauge error GE times the sum of the ratio of the workpieceplasticity P for that same stand with the mill spring modulus K for thatsame stand and one, as follows in relation to stand (N):

ASD(N) =exit GE(N) [(P(N)/K(N)) 1 ferred in terms of inches of screwdownposition change per millions of pounds of roll force. Thusly, the aboveequation (10 will be rewritten and utilized in accordance with therelationship:

A SD(N) exit GE(N) [K(N)/ +1 The stand (N) exit gauge error GE(N)determined by the roll force system at stand (N) is established by therelationship of above equation (ll) as follows:

In reference to FIG. 4, in general the workpiece strip gauge errordelivered by a given stand, and as determinedby the sensed operationalvariable at that same stand, is in accordance with the roll force systemrelationship shown in above euqation (12). Th exit gauge error leavingstand (N), for example, equals the sum of a first quantity, which is thedifference between the presently measured screwdown position LOSD(N),and a seocnd quantity, which is the determined mill spring modulus K(N)times the difference between the presently measured roll separationforce P(N) and the initial lock on roll force LOF(N).

DESCRIPTION OF-EMBODIMENT OF PRESENT INVENTION In reference to FIG. 5,ther is shown a portion of a tandem rolling mill including a last rollstand (LS), and an earlier roll stand (N), with the workpiece strip 15moving in the direction indicated by the arrow. At block 400 there isdetermined the roll force exit gauge error leaving stand (N) in relationto the operational variables sensed at stand (N). This determinationutilizes above Equation (12) for this purpose. At block 402 there isdetermined the stand (N) X-ray gauge error XGE(N) leaving stand (N).

The roll force gauge control system maintains substantially constantdelivery gauge out of reach roll stand in relation to the initiallysetup lock on gauge at the head-end of each workpiece strip. The X-raygauge sensing device located after the last roll stand is used todetermine the X-ray delivery gauge deviation leaving the rolling mill,in relation to the measured actual gauge and the desired referencegauge. The particular roll stands selected by the operator for X-raymonitor correction are adjusted in operation to bring the final deliverygauge or thickness leaving the rolling mill into agreement with thedesired refernce gauge, if the X-ray gauge deviation is not too large.The measured output X-ray gauge deviation from the X-ray device isprocessed by the following Equation relationship:

XGE(N) X-ray Deviation [(S(LS/S(N))] [(Jl/J2+ OLDXGE(N) 13) nos/14)] todetermine the X-ray gauge error XGE(N) for stand (N) in relation to themeasured speed S(LS) of the last stand, the measured speed S(N) of stand(N), a first preselected adjustment parameter J 1/12 for stand (N), theprevious determined value of the X-ray gauge error OLDXGE(N-) for stand(N) in accordance with this same Equation (13) and a secondpreselected'adjustment parameter .l3/J4 for stand (N).'The speed S(N) ofstand (N) and the speed S( LS) of the last stand are measured inrelation to the operational speed of these respective roll stands, sincethe forward slip consideration at stand (N) balances the forward slipconsideration at the last stand (LS). Assuming stand (N) is selected forX-ray monitor correction, at block. 404 the X-ray gauge error XGE(N) atstand (N) is utilized in combination with the roll force determined exitgauge error GE(N) to establish the desired screwdown position adjustmentASD(N) in accordance with the relationship of above Equation (1 l)modified as fo llowsi At block 410 there is determined the exit gaugeerror leaving last stand (LS) in relation to the operational variablessensed at the last stand (LS), and this utilizes above Equation (5) forthis purpose. At block 412 there is determined the calculated last stand(LS) X-ray gauge error XGE(LS) for the last stand (LS), and thisutilizes above Equation (13) for this purpose. At block 414 there isdetermined the last stand (LS) screwdown position correction needed toremove both the exit gauge error GE(LS) at the last stand (LS) as wellas the X-ray gauge error XGE(LS) at the last stand (LS), and thisutilizes above Equation (13) for this purpose with the ratio of S(LS)/S(LS) being used here.

In relation to the X-ray gauge error correction, there is shown in FIG.6 a flow chart to illustrate the operation of this program. At step 600a check is made to see that the operator has selected the X-ray monitoroperation to be operative. At step 602 a check is made to see that aparticular X-ray device is selected for operation in the event that twoX-ray devices are provided after the last stand. At step 604 adetermination is made that the selected X-ray device is measuring stripgauge. If any one of the determinations at steps 600, 602 and 604 isnegative then the program ends. At step 606 the operator desired targetor nominal workpiece strip gauge leaving the rolling mill is read fromstorage. It should be understood that gauge is herein used to mean thesame as workpiece strip thickness, and it is commonly also spelled gageby persons skilled in this art. At step 608 the percent deviationbetween the desired nominal or reference gauge and the X-ray devicemeasured actual gauge is now determined. At step 610 a limit checkis-made, and if it is too large a flag is set and an alarm messageprinted at step 6l2 and the rogram ends. If the percent deviation is nottoo large, at step 614 a check is made to see if the head-end time delayhas expired; and if it has not the program ends. At step 618 adetermination is made to see if this is the first check on this strip.If it is, at step 620 a check is made to see if monitor hold is selectedby the operator and if so at step 622 the present gauge is held. lf thecheck at step 618 was negative, the program goes to step 624 to set thedrive number equal to last stand. If the check at step 620 was negative,the program goes to step 626 to determine if the gauge deviation gaugeerror is the maximum allowable. At step 628 a selection is made of theclosest alternate gauge from the stored gauge table provided by theoperator. From step 622 the program goes to step 630 to calculate a newpercent deviation. At step 632 the monitor hold light is turned on. Thecomparison made at step 626 is provided to determine if the percentdeviation is greater than some operator predetermined limit value, suchas 10 percent. At step 628 a look-up table operation is provided inrelation to operator provided values to reapply the desired or nominalstrip gauge. At step 630 a new percent deviation is determined inrelation to this new desired strip gauge.

At strip 624 the drive number is set equal to the last stand inpreparation of determining the last stand speed and a mass flowrelationship including proportional integration of the established gaugeerror to be performed on a selected stand by stand basis, generallythree such stands are selected by the operator. At step 624 the laststand is addressed, and now the correction of the selected stand occurs.At step 626 a check is made to see if the selected stand has calibratedscrews, and at step 628 a check is made to see if the X-ray monitoroperation has been selected by the operator for this stand. At step 630the X-ray correction is determined for the selected stands in accordancewith above Equation (13), including the proportional integrationfunction. This operation is continued for all selected stands. If thechecks made at step 626 or 628 are failed, then the stand drive numberis decremented at step 632 and a check is made at step 634 to see ifthis stand is number zero. At steps 636 and 638 the correction islimited. At step 640 if the stand roll force gauge control system isturned off, at step 642 the present screw position is read and an X-raycorrection is output for this stand at step 644; this permits providingonly the X-ray correction with the roll force system turned off for agiven stand when desired by the operator. At step 646 a check is made tosee if enough stands have been corrected. At step 634 a check is made tosee if this stand under consideration is the first stand and at step 634the stand number is decremented to continue the operation for allselected stands.

The typical AGC control program is written as a loop operation such thatone set of coding processes all of the roll stands, and every time theprogram operates thrugh the loop a calculation is made when appropriatefor each of the roll stands in relation to the gauge error and the X-raygauge error correction.

The following table shows illustrative values of the first adjustmentfactor .Il/J2 as utilized in relation to above Equation (13) as well asthe second adjustment factor 13/14 when plotted in relation to therespective stand numbers of a typical tandem rolling mill.

Stand No.

Jl/JZ J3IJ4 1 0.40 1.00 2 0.50 1.00 3 0.60 1.00 4 0.70 1.00 5 0.80 1.006 0.90 1.00 7- 1.00 1.00

The above Equation (13) relationship operates on the X-ray gaugedeviation as a proportional integrator, such that the first term of theEquation provides a substantially instantaneous response to changes inthe X-ray gauge deviation measured by the X-ray device, while the secondterm of the Equation provides an integral response to the long termtrends of the X-ray gauge deviation measured by the X-ray device.

Previous gauge control systems used the X-ray device to measure thedeviation in gauge from the nominal selected value of desired deliverygauge by a comparison of the actual delivery gauge with the desiredtarget delivery gauge to give this gauge deviation. This gauge deviationwas applied as an offset recalibration to the screw position reading.The present control arrangement takes a different approach, bydetermining the X-ray correction and applying the correction to the rollforce gauge control Equation as an additional term in relation to thegauge error. The previous offset was not GENERAL DESCRIPTION OFINSTRUCTION PROGRAM LISTING In the Appendix there is included aninstruction program listing that has been prepared to control the rollforce automatic gauge control operation of a tandem rolling mill inaccordance with the here disclosed control system and method. Theinstruction program listing is written in the machine language of thePRODAC P2000 digital computer system, which is sold by WestinghouseElectric Corporation for real time process control computerapplications. Many of these digital computer systems have already beensupplied to customers, including customer instruction books anddescriptive documentation to explainto persons skilled in this art theoperation of the hardware logic and the executive software of thisdigital computer system. This instruction program listing is included toprovide an illustration of one suitable embodiment of the presentcontrol system and method that has actually been prepared. Thisinstruction program listing at the present time has been partiallydebugged through the course of practical operation for the real timeautomatic gauge control of a tandem rolling mill, but it is understoodand well known by persons skilled in this art that most real timeprocess control application programs contain some bugs or minor errors,and it is within the routine skill of such persons and takes varyingperiods of actual operation time to identify and correct the morecritical of these bugs.

A person skilled in the art of writing computer instruction programlistings, particularly for an invention such as the present roll forceautomatic gauge control system and method for a tandem rolling mill mustgenerally go through the following determinative steps:

Step One Study the workpiece rolling mill and its operation to becontrolled, and then stablish the e q n wlsy sm and. sthsqwn spt StepTwo Develop an understanding of the control system logic analysis,regarding both hardware and software.

Step Three Prepare the system flowcharts and/or the more detailedprogrammer's tlowcharts.

Step Four Prepare the actual computer instruction P o ra t rom, it? tiershar a What we claim is: t

I. A gauge control system for a rolling mill having at least one rollstand (N) operative to reduce the gauge of a workpiece passed throughsaid roll stand and including a device for measuring the gauge deviationof the workpiece leaving said rolling mill, said system comprising:

means for determining a gauge error of said workpiece leaving said oneroll stand in relation to said measured gauge deviation, means operativein relation to said gauge error for determining the required adjustmentof said one roll stand in accordance with a predetermined relationshipincluding the mill spring modulus of said one roll stand and theworkpiece plasticity in relation to said one roll stand, and means forcontrolling the operation of said one roll stand in accordance with saidrequired adjustment.

2. The gauge control system of claim 1, with said predeterminedrelationship being as follows:

A so XGE [(K/P) .1

where A SD is the required adjustment of said one roll stand,

where XGE is the gauge error of said one roll stand in relation to saidgauge deviation,

where K is the mill spring modulus of said one roll stand, and

where P is the workpiece plasticity of said one roll stand.

3. The gauge control system of claim 1, including means for determininga second gauge error of said workpiece leaving said one roll stand inrelation to the roll force, and screwdown position of said one rollstand,

with said means for determining said required adjustment being operativein relation to said second gauge error. 4. The gauge control system ofclaim 3, with said predetermind relationship being as follows where ASD(N) is the required adjustment for said one roll stand (N),

where GE(N) is said second gauge error,

where XGE(N) is the gauge error in relation to said gauge deviation,

where K(N) is the mill spring modulus of said one roll stand (N), and

where P(N) is the workpiece plasticity in relation to said one rollstand (N).

5. A gauge control system for a rolling mill having at least a firstroll stand and a last roll stand operative with respective initial rollopening settings to reduce the gauge of a workpiece passed through saidrolling mill and including a device positioned after said last rollstand for measuring the gauge deviation of said workpiece leaving saidrolling mill, said system comprising:

means for determining a guage error of said workpiece in accordance witha predetermined relationship between said gauge deviation of saidworkpiece, the operating speed of said first roll stand and theoperating speed of said last roll stand and a response factor,

means for determining a correction to the roll opening setting of atleast said first roll stand in accordance with said gauge error, themill spring modulus of said first roll stand and the workpieceplasticity, and

means for controlling the roll opening of said first roll stand for thepassage of said workpiece in accordancewith said correction.

6. The gauge control system of claim 3, with said predeterminedrelationship being as follows:

XGE(N) Gauge Deviation (S(LS)/S(N)) RF(1) OLDXGE(N) RF(2) where XGE(N)is a gauge error at said first roll stand (N) in relation to said gaugedeviation,

where S(LS) is the speed of the last stand,

where S(N) is the speed of the first stand (N),

where RF( 1 is a first predetermined response factor,

where OLDXGE(N) is the integral of the gauge error XGE(N) for thisworkpiece, and where RF (2) is a second predetermined response factor.7. A gauge control system for a rolling mill having a plurality of rollstands operative to reduce the gauge of 'a workpiece passed through eachof said roll stands and including a device for measuring the gaugedeviation of the workpiece leaving said rolling mill, said systemcomprising:

means for determining a gauge error of said workpiece leaving each saidroll stand in relation to said measured gauge deviation, means operativein relation to said gauge error for each roll stand for determining arespective required adjustment for each said roll stand in accordancewith a predetermined relationship including the mill spring modulus ofthe same roll stand and the workpiece plasticity in relation to the sameroll stand, and I means for controlling the operation of each said rollstand in accordance with its respective required adjustment. 8. Thegauge control system of claim 7, with said required adjustment for eachroll stand (N) being as follows:

where ASD(N) is the required adjustment of each said roll stand (N),

where XGE(N) is the gauge error in relation to each roll stand (N),

where K(N) is the mill spring modulus in relation to each roll stand(N), and

where P(N) is the workpiece plasticity in relation to each roll stand(N).

9. A method of controlling the workpiece gauge leaving a rolling millhaving at least one roll stand operative with an initial roll openingsetting to reduce the gauge of a workpiece passed through said rollingmill and including a device for measuring the gauge deviation of saidworkpiece .leaving said rolling mill, the steps of said methodcomprising:

determining a gauge error of said workpiece leaving said one roll standin relation to the measured gauge deviation and a predetermined responsefactor,

determining a roll opening correction fr application to said one rollstand during the passage of said workpiece in accordance with apredetermined relationship including said gauge error, the mill springmodulus of said one roll stand and the workpiece plasticity in relationto said one roll stand, and

controlling the operation of said one roll stand in accordance with saidroll opening correction.

10. The method of claim 9, with the gauge error being determined by thefollowing relationship:

XGE(N) X-ray Deviation (S(LS)/S(N)) RF(1) OLDXGE(N) RF(2) whereOLDXGE(N) is the integral of the gauge error in relation to said oneroll stand (N), and where RF(2) is a second predetermined responsefactor.

1. A gauge control system for a rolling mill having at least one rollstand (N) operative to reduce the gauge of a workpiece passed throughsaid roll stand and including a device for measuring the gauge deviationof the workpiece leaving said rolling mill, said system comprising:means for determining a gauge error of said workpiece leaving said oneroll stand in relation to said measured gauge deviation, means operativein relation to said gauge error for determining the required adjustmentof said one roll stand in accordance with a predetermined relationshipincluding the mill spring modulus of said one roll stand and theworkpiece plasticity in relation to said one roll stand, and means forcontrolling the operation of said one roll stand in accordance with saidrequired adjustment.
 2. The gauge control system of claim 1, with saidpredetermined relationship being as follows: Delta SD XGE * ((K/P) + 1)where Delta SD is the required adjustment of said one roll stand, whereXGE is the gauge error of said one roll stand in relation to said gaugedeviation, where K is the mill spring modulus of said one roll stand,and where P is the workpiece plasticity of said one roll stand.
 3. Thegauge control system of claim 1, including means for determining asecond gauge error of said workpiece leaving said one roll stand inrelation to the roll force and screwdown position of said one rollstand, with said means for determining said required adjustment beingoperative In relation to said second gauge error.
 4. The gauge controlsystem of claim 3, with said predetermined relationship being as followsDelta SD(N) (GE(N)+XGE(N)) * ((K(N)/P(N)) + 1) where Delta SD(N) is therequired adjustment for said one roll stand (N), where GE(N) is saidsecond gauge error, where XGE(N) is the gauge error in relation to saidgauge deviation, where K(N) is the mill spring modulus of said one rollstand (N), and where P(N) is the workpiece plasticity in relation tosaid one roll stand (N).
 5. A gauge control system for a rolling millhaving at least a first roll stand and a last roll stand operative withrespective initial roll opening settings to reduce the gauge of aworkpiece passed through said rolling mill and including a devicepositioned after said last roll stand for measuring the gauge deviationof said workpiece leaving said rolling mill, said system comprising:means for determining a gauge error of said workpiece in accordance witha predetermined relationship between said gauge deviation of saidworkpiece, the operating speed of said first roll stand and theoperating speed of said last roll stand and a response factor, means fordetermining a correction to the roll opening setting of at least saidfirst roll stand in accordance with said gauge error, the mill springmodulus of said first roll stand and the workpiece plasticity, and meansfor controlling the roll opening of said first roll stand for thepassage of said workpiece in accordance with said correction.
 6. Thegauge control system of claim 3, with said predetermined relationshipbeing as follows: XGE(N) Gauge Deviation * (S(LS)/S(N)) * RF(1)+OLDXGE(N) * RF(2) where XGE(N) is a gauge error at said first rollstand (N) in relation to said gauge deviation, where S(LS) is the speedof the last stand, where S(N) is the speed of the first stand (N), whereRF(1) is a first predetermined response factor, where OLDXGE(N) is theintegral of the gauge error XGE(N) for this workpiece, and where RF(2)is a second predetermined response factor.
 7. A gauge control system fora rolling mill having a plurality of roll stands operative to reduce thegauge of a workpiece passed through each of said roll stands andincluding a device for measuring the gauge deviation of the workpieceleaving said rolling mill, said system comprising: means for determininga gauge error of said workpiece leaving each said roll stand in relationto said measured gauge deviation, means operative in relation to saidgauge error for each roll stand for determining a respective requiredadjustment for each said roll stand in accordance with a predeterminedrelationship including the mill spring modulus of the same roll standand the workpiece plasticity in relation to the same roll stand, andmeans for controlling the operation of each said roll stand inaccordance with its respective required adjustment.
 8. The gauge controlsystem of claim 7, with said required adjustment for each roll stand (N)being as follows: Delta SD(N) XGE(N) * ((K(N)/P(N)) + 1) where DeltaSD(N) is the required adjustment of each said roll stand (N), whereXGE(N) is the gauge error in relation to each roll stand (N), where K(N)is the mill spring modulus in relation to each roll stand (N), and whereP(N) is the workpiece plasticity in relation to each roll stand (N). 9.A method of controlling the workpiece gauge leaving a rolling millhaving at least one roll stand operative with an initial roll openingsetting to reduce the gauge of a workpiece passed through said rollingmill and including a device for measuring the gauge deviation of saidworkpiece leaving said rolling mill, the steps of said methodcomprising: determining a gauge error of said workpiece leaving saiD oneroll stand in relation to the measured gauge deviation and apredetermined response factor, determining a roll opening correction forapplication to said one roll stand during the passage of said workpiecein accordance with a predetermined relationship including said gaugeerror, the mill spring modulus of said one roll stand and the workpieceplasticity in relation to said one roll stand, and controlling theoperation of said one roll stand in accordance with said roll openingcorrection.
 10. The method of claim 9, with the gauge error beingdetermined by the following relationship: XGE(N) X-ray Deviation *(S(LS)/S(N)) * RF(1) +OLDXGE(N) * RF(2) where XGE(N) is the gauge errorleaving said one roll stand (N), where S(LS) is related to the speed ofthe workpiece leaving the rolling mill, where S(N) is related to thespeed of the workpiece leaving said one roll stand (N), where RF(1) is afirst predetermined response factor, where OLDXGE(N) is the integral ofthe gauge error in relation to said one roll stand (N), and where RF(2)is a second predetermined response factor.